Propylene homopolymers have been difficult to process on blown film extrusion equipment, since the inherent low melt strength of these polymers causes bubble instability and poor film gauge control. The range of processing conditions for polypropylene is quite narrow and difficult to control. The addition of an ethylene copolymer, such as an ethylene/propylene copolymer, aids in control of the film bubble, since ethylene homopolymers and copolymers process well on this type of equipment. However, the processing of such copolymers is still quite sensitive.
U.S. Pat. No. 5,318,842 discloses a film or sheet comprising (a) a film or sheet of a crystalline homopolymer of a 3-10 carbon alpha-olefin monomer or certain propylene copolymers, and (b) at least one surface layer of a broad molecular weight distribution propylene polymer material. The multilayer films are coextruded and biaxially oriented. U.S. Pat. No. 5,409,992 discloses blends of (a) a heterophasic olefin polymer material, and (b) a broad molecular weight distribution propylene polymer material, and their use for making calendered films and sheets. Neither of these references discloses the manufacture of blown film from broad molecular weight distribution propylene polymer materials.
A propylene polymer that could be used to produce blown film with a stable film bubble and the capability for tight film gauge control would be desirable.